For the presentation of various transmission ratio stages, automatic transmissions feature hydraulic shift elements, which are actuated by means of pressurization; that is, they are brought into an operating state in which a certain torque is transferable through the shift elements. For this purpose, a cylindrical space is to be filled with an operating medium, and subsequently a clutch piston is subjected to a certain pressure of the operating medium. The volume flow or pressure required for filling or pressurization is generated by a pump, which is driven by a drive unit of the automatic transmission. The amount of pressure is adjusted by a pressure adjusting device, whereas different pressure circuits may be present in the hydraulic system of a transmission. When the drive unit is at a standstill, the generation of pressure collapses, and the pressure in the hydraulic system of the automatic transmission decreases to zero or the ambient pressure, thus also in the hydraulic shift elements. In addition, the cylinder of the shift elements is at least partially emptied, such that, upon a renewed starting of the drive unit, the cylinders must initially be completely filled before the shift elements are actuated as desired and a transmission ratio stage may thus be engaged.
DE 198 58 541 A1 shows an electronic/hydraulic control device of a motor vehicle transmission to be automatically shifted with hydraulically actuated shift elements. The pressure source of the hydraulic system is a pump, which provides a volume flow of pressure medium, whereas the amount of the pressure is adjusted in the pressure medium by means of valves acting as pressure adjusting devices. The filling of each shift element is triggered by raising the switching pressure, which moves a piston slide of a clutch valve into a switching position, in which the clutch is filled through the switching valve from ultimately the pressure source. If the drive unit that drives the pump is turned off and the control device is depressurized, the clutch valve is moved to a position at which the hydraulic connection of the clutch to the pressure source is interrupted, and a hydraulic connection of the clutch to a check valve is established. The check valve prevents the emptying of the clutch after, due to the outflow from the clutch through the check valve to an unpressurized chamber of the transmission, the clutch pressure has been reduced to a certain residual pressure value. By doing so, a defined residual pressure value in the clutch is adjusted. Ambient pressure prevails under a non-pressurized chamber, hereinafter referred to as a vented area.
The disadvantage here is that the residual pressure in the clutch is gradually eliminated through the gap leakage caused by its design, and that the clutch can thus be drained, since the clutch valve is designed as a piston slide valve. In addition, the valve device consists of a clutch valve for each shift element and a separate check valve, by which a high number of parts and additional installation space with a corresponding channel guide is required.